[ARC5] [Milsurplus] Cold Filament Inrush Current

Thu Mar 15 16:04:08 EDT 2018

There was a Western Electric (IIRC) patent from back around the late 40’s regarding a resistor keeping lamp filaments preheated as a way of extending life and improving response time. 

Don’t forget to turn it off in a battery operated device though!

Peter

> On Mar 15, 2018, at 3:54 PM, Tim <timsamm at gmail.com> wrote:
> 
> In a similar vein I once modified an old "right angle" military flashlight with a key jack in series with the D Cells.  (the built in push button "key" switch would kill your finger very quickly)
> 
> As a "flashing light" training device in the Navy.  Didn't work very well as you could imagine.  So I put a resistor across the key for some keep-alive current and the light rise-time then worked very well for morse training.  Hooyah!
> 
> 73, Tim
> N6CC
> 
>> On Thu, Mar 15, 2018 at 12:45 PM, Bobby Drummond <alphak4ja at gmail.com> wrote:
>> Here is my poor man's answer to inrush filament current potential problems (especially with series string filaments)
>> 
>> I power the rig first through a 25 watt incandescent light bulb (bulb in series with the 120 volt power to the rig) for 30 seconds (long enough for the filaments to warm sufficiently at the lower voltage allowed through the series incandescent light bulb), disconnect and then quickly power the rig with full normal voltage.
>> 
>> It's the cheapest way I know of the reduce the effects of initial filament inrush current on the rigs I use.
>> 
>> 
>> 73 de AK4JA
>> 
>> 
>>> On Thu, Mar 15, 2018 at 3:33 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:
>>> Yes, blooming filaments = bad!
>>> 
>>> If you are using the gear pretty heavily and are concerned about the on-off stress on filaments, there are several solutions. NTCs have already been mentioned, and are probably the most expedient solution in most cases.
>>> 
>>> Broadcast transmitters generally keep the filaments warm when not in use. Since the lifetime of a filament is proportional to something craxy, like the 12th (!) power of voltage, it doesn't take much voltage reduction to prolong lifetime to whatever value you want. Then the shock of going full out is moderated, taking care of that concern.
>>> 
>>> "Instant-on" TVs simply inserted a single rectifier in series with the filament string so that the tubes were kept warm even when the set was off. The reduced filament temperature had negligible impact on tube lifetime. Bridging across the rectifier turned the set on "right away" without much thermal stress. This trick might be a viable option for some usage scenarios.
>>> -- 
>>> Prof. Thomas H. Lee
>>> Allen Bldg., CIS-205
>>> 420 Via Palou Mall
>>> Stanford University
>>> Stanford, CA 94305-4070
>>> http://www-smirc.stanford.edu
>>> 650-725-3383 (public fax; no confidential information, please) 
>>>> On 3/15/2018 12:22 PM, DSP3 wrote:
>>>> One of the factors mentioned in the Eimac power tube book is to address in-rush by ensuring that the filament transformer (especially in thoriated tungsten types) is rated for the actual current of the tubes, and no more.  That way the lower, in-rush resistance it initially mitigated by sag in the transformer.  The opposite scenario is that an overrated transformer will sink whatever current necessary during the cold, heat-up period.  I have seen tubes "bloom" when power is applied where the extra current is available.  Not a good situation...
>>>> 
>>>> Jeep - K3HVG
>>>> 
>>>>> On 3/15/2018 2:37 PM, Tom Lee wrote:
>>>>> Exactly. Resistance is a function of temperature, so tubes that operate at white-hot incandescence (pure tungsten) will have a larger hot-to-cold resistance ratio than oxide-cathode tubes. For the former, a 5:1 ratio is not uncommon, where for the latter, something around 2:1 is more typical. 
>>>>> 
>>>>> --Tom 
>>>>> 
>>>> 
>>>> 
>>>> 
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>> 
>> 
>> -- 
>> 73 de AK4JA
>> 
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